1. Field of the Invention
The present invention relates generally to switching power converter circuits, and more specifically, to a switching power converter in which an auxiliary winding power supply and a current sensing circuit receive input from the same current sensing transformer secondary winding.
2. Background of the Invention
Operation of switching power converters in continuous conduction mode (CCM) is desirable for efficiency and to reduce the peak current levels in the converter for reliability and reduction of component costs. In CCM operation, the current through the magnetic coupling element never changes polarity and typically never fully drops to zero. Since the current is never permitted to reach zero, saturation of the magnetic coupling element can occur if an increasing magnetic flux progressively accumulates in the magnetic coupling element at the end of each switching cycle, a phenomenon sometimes referred to as “flux walking.” Eventually the core of the magnetic coupling element will saturate, causing the magnetic coupling element to appear as a short circuit, resulting in failure of the switching circuits. Therefore, in CCM operation, detection of the peak current, or another reference current level, through the series circuit formed by the magnetic storage element and the switching circuit is needed. The peak current value determines the off cycle period needed to discharge the inductor. In discontinuous conduction mode (DCM), there is always a period of zero current flowing in the magnetic coupling element, and therefore the core of the magnetic coupling element always resets to zero at each cycle.
In order to sense the current through the magnetic coupling element, a sense resistor may be inserted into the series switching circuit, or an additional current sensing transformer can be provided. The current sensing transformer approach is typically more efficient than the approach of adding a current sense resistor, as the transformer and current sensing circuit are designed to produce minimal disturbance and loss in the power switching circuit. A current sensing transformer primary winding typically includes only a few turns, so the voltage across the primary winding is small. The secondary winding of the current sensing transformer generally produces a voltage on the order of a volt or less, so that the magnetization current of the current sensing transformer (also referred to as the magnetizing current) can be disregarded. Therefore, the secondary current of the typical current sense transformer is substantially proportional to the primary current, so that a voltage generated across a resistor connected across the secondary of the current sensing transformer is linearly related to the primary current and provides a measure of the current passing through the primary winding of the main magnetic coupling element of the converter.
In order to supply power to control circuits of a switching power converter, a low voltage power supply is needed. When the only voltage available or convenient is a high-voltage power supply, or when isolation of the auxiliary power supply is required, an auxiliary winding on the main magnetic coupling element of the switching converter is commonly used to supply power to the control circuits. However, including an auxiliary winding increases the cost of the magnetic coupling element. In contrast to the characteristics of the secondary winding in the above-described current sensing transformer, as the auxiliary power supply typically must provide a voltage on the order of 10V and also generate substantial current, on the order of tens or hundreds of milliamps. Therefore, an auxiliary winding generally has substantial magnetization current, and the voltage across the typical auxiliary winding does not provide an accurate indication of the current flowing through the primary winding of the main magnetic coupling element of the switching power converter.
Therefore, it would be desirable to provide an auxiliary power supply in a switching power converter having a current sense transformer without requiring an auxiliary winding on the main magnetic coupling element of the switching power converter.